A conventional subsea horizontal tree includes a wellhead housing which contains one or more casing hangers, one of which is secured to a string of production casing that extends into the well. A horizontal tree body, also known as a tree or tubing spool or spool tree, mounts to the top of the wellhead housing and seals to it. The horizontal tree body has a central bore axially through it and a horizontal or lateral production flow passage through the wall of the horizontal tree body. A tubing hanger lands and seals in the central bore of the horizontal tree body and is secured to a string of tubing that extends through the production casing hanger and production casing into the well. The tubing hanger has a production bore axially through it that is in fluid communication with the tubing. The tubing hanger also has a lateral flow passage in fluid communication with the tubing hanger production bore and with the lateral production flow passage in the horizontal tree body.
Annular seals are located between the tubing hanger and the central bore of the horizontal tree body above the production flow passage to provide primary, and occasionally, secondary barriers to leakage from the production flow pathways and well bore. Additionally, one or more wireline deployable plugs fit in one or more lockdown profiles in the tubing hanger production bore to provide primary, and occasionally, secondary barriers to leakage from the production and well bores. A tree cap may also fit above the tubing hanger in the central bore of the horizontal tree body. The tree cap may be of an internal or external lockdown configuration. In either case, the tree cap will seal to the central bore of the horizontal tree body and act as an additional barrier to leakage from the well. The tree cap of either configuration may have a vertical bore through it.
Another typical feature of subsea horizontal trees is an annulus and workover passageway that establishes a fluid communication pathway between the annular space around the tubing below the tubing hanger and a space inside the central bore of the horizontal tree body above the tubing hanger. This annulus and workover passageway can be ported through the tubing hanger, through the horizontal tree body or a combination of both. Alternatively, the annulus and workover passageway may be ported entirely out of the tree from a position below the tubing hanger.
In practice, there are generally two horizontal tree configurations: (1) a horizontal tree with a tubing hanger fitted with one or more plugs in its production bore and an internal tree cap, with a plug in its vertical bore; or (2) a horizontal tree with a tubing hanger fitted with at least two plugs in its production bore and eliminating the internal tree cap. This second style of horizontal tree typically utilizes a tree cap that locks externally to the tree body and may or may not include a seal to the tree body. In either tree cap case, the annulus and workover passageway will contain at least two closure members in the form of gate valves, for example.
The primary difference between these two general horizontal tree configurations is that the first has a primary and secondary barrier that employs independent lockdown structures for the two barriers, and the second has a primary and secondary barrier that ultimately rely on the tubing hanger to horizontal tree body lockdown structure. Some operators, and some regulatory authorities believe that the first and second horizontal tree configurations are equivalently safe in operation. Other operators and regulatory authorities believe only the first configuration meets the dual barrier industry philosophies and/or regulatory requirements.
One advantage of the second configuration is that the elimination of the internal tree cap eliminates the need for a second drill pipe run to install it. In running (or working over) the first style of horizontal tree, the tubing hanger is run into the horizontal tree body typically on a hydraulically-actuated running tool that is run on drill pipe. Afterwards, the internal tree cap is run into the horizontal tree body typically on the same hydraulically-actuated running tool, or one very similar, on drill pipe. This results in two drill pipe trips to the seafloor.
In running (or working over) the second style of horizontal tree, the tubing hanger is run into the horizontal tree body typically on the hydraulically-actuated running tool on drill pipe. Afterwards, a lower plug is run on wireline and landed, locked and sealed to the production bore of the tubing hanger and then an upper plug is run on wireline and landed, locked and sealed to the production bore above the first plug. In deeper water wells, this results in potentially significant rig time savings. However, it comes with the compromise that the two plugs rely on the single tubing hanger lockdown mechanism to ensure that the tubing string assembly does not part from the tree and cause potentially significant leakage of the well bore to the environment.